Smooth gauge needle animation
Assumption:

The speedometer does not move or rotate. It stays at the same place in scene.

Design:

Animate an invisible object (axis) that turns according to the input values.

Parent a visible indicator (needle) to the axis. Enable slow parent (Properties editor / object tab / Relation Extras panel) with an offset > 0. 

Effect:

while the axis immediately shows the received value the needle will smoothly go for that pose.